The present invention relates to micro-actuator devices, and more particularly to a system and method for multi-axis controlled rotation of micro-scale payloads using cascaded electrothermal microactuators.
Using two materials to create an actuator has been known for decades. A bimetal strip, for example, will change its geometry with an increase in temperature due to a difference in thermal expansion coefficients.
Thermal actuators made of a single material have been described in the prior art. One bimorph embodiment, known as a Heatuator, is based on preferential expansion of one beam, which gets hotter than the other due to its smaller geometric cross-sectional area. These actuators require substantial energy to deflect, whereas other bimorph embodiments, which depend on the difference of properties between two dissimilar material layers and not on a preferential heating scheme, operate at lower temperatures with lower currents. For example, Tuantranont, et al., xe2x80x9cSmart Phase-Only Micromirror Array Fabricated By Standard CMOS Process,xe2x80x9d IEEE 0-7803-5273-4/00 (2000) pp. 455-460 describes a piston micro-mirror incorporating four bimorphs in a standard CMOS foundry process providing a metalization layer, which is usually used only to conduct signals, on top of an oxide layer or a polysilicon layer involved in the process. Generally, bimorph configurations include a beam which has two different material layers and achieves end deflection relatively small compared to the length of the beam. Resistive joule heating is used to actuate the device. When current passes through it, it heats and deflects slightly. One problem is a limited range of motion. Another problem is difficulty obtaining rotary motion about an axis.
Prior art rotary devices incorporate multiple actuators that, for instance, engage a released circular plate and step it in small rotary increments. For example, Sarkar, et al., xe2x80x9cActuator Design for Variable Capacitors and Optical MEMSxe2x80x9d presented at the Canadian Workshop on MEMS/Micromachining: Applying MEMS Research in Canada, Ottawa, Ontario (Aug. 17, 2001), hereby incorporated herein by reference, describes the use of Heatuators to grip and rotate a circular gear in a stepped fashion.
One method of scanning an optical beam requires mechanical rotation of a reflective surface. For example, Hornbeck, et al., xe2x80x9cDigital Micromirror Device(trademark)xe2x80x94Commercialization of a Massively Parallel MEMS Technology,xe2x80x9d American Society of Mechanical Engineers, (DSC-Vol. 62, 1997), pp. 3-8, describes a tilting mirror supported and actuated by an electrostatically-driven torsion bar. The angle about which the mirror can rotate is limited in this approach to approximately 15 degrees. A related micromechanism is described in Toshiyoshi, et al., xe2x80x9cElectrostatic Micro Torsion Mirrors for an Optical Switch Matrix,xe2x80x9d IEEE J. Microelectromechanical Systems, Vol. 5, No. 4 (Dec. 1996), pp. 231-237.
Motamedi, et al., xe2x80x9cMicro-opto-mechanical Devices and On Chip Optical Processing,xe2x80x9d Opt. Eng. Vol. 38, No. 5 (May 1997), pp. 1282-1297, describes a micro optical bench on which mirrors on a micron scale are hinged to a substrate and rotated up to 90 degrees (i.e., perpendicular to the substrate), using bent beam or scratch drive actuators. These devices have also been designed to allow the mirror to return parallel to the substrate, thereby creating a rotational mirror device. It has also been demonstrated that several such mirrors can be configured on the substrate to create an optical switch by timing the flip up action.
Magnetically actuated self erecting structures have been described generally in the prior art. For example, Judy, et al., xe2x80x9cMagnetically Actuated, Addressable Microstructures,xe2x80x9d IEEE J. Microelectromechanical Systems, Vol. 6, No. 3 (Sep. 1997), pp. 249-256, describes using a large magnetic field on the outside to activate micro devices made, for instance, of permalloy material. When the external magnetic field is applied, the magnetic material aligns itself in a new orientation, which is dependent on the field geometry. Pannu, et al., xe2x80x9cClosed-Loop Feedback-Control System for Improved Tracking in Magnetically Actuated Micromirrors,xe2x80x9d IEEE 0-7803-8/00 (2000), pp. 107-108, describes controller embodiments that improve dynamic response and positioning precision of magnetically actuated micromirrors.
The prior art teaches multiple ways of cascading actuators to amplify motion. For instance, bent beam actuators produce small deflection in one direction if they are anchored on both sides. Que, et al., xe2x80x9cBent-Beam Electrothermal Actuatorsxe2x80x94Part I: Single Beam and Cascaded Devices,xe2x80x9d IEEE J. Microelectromechanical Systems, Vol. 10, No. 2, (Jun. 2001), pp. 247-254, describes cascading of bent beams by aligning two bent beam actuators at a selected angle relative to a third bent beam actuator. Small deflections at the tips of the two outside actuators push on the base of the third actuator to further amplify the motion at the tip of the third actuator, thereby creating a slightly larger deflection.
It would be desirable in the art to have some mechanism using bimorphs to generate large angle rotary motion as opposed to just linear motion, through actuation of linear tip displacements.
The present invention is directed to a system and method which cascade linear bimorph actuators to achieve large angle rotary displacements. Bimorph units contain substantially parallel pairs of beams, including a single material beam that remains straight when heated and a bilayer beam that deflects when heated, due to differential thermal expansion of the layers. In some embodiments, this concept is applied as part of a unit cell. For a bilayer beam, advantageous materials are gold on top of polysilicon. As the bilayer beam is heated, the metal expands more than the polysilicon, producing a deflection at the end of this beam. The angular deflection is amplified by mechanically cascading interconnected unit cells in a serpentine fashion. In some embodiments, successive beams are connected electrically in series to provide a continuous current path for resistive joule heating of the beams. This configuration achieves cumulative rotational displacements up to greater than 90 degrees. In some embodiments, the actuator is fully released and removed from the substrate to prevent mechanical interference against the substrate when actuated. In other embodiments, at least a segment of the substrate is removed from beneath the actuator to prevent interference. In further embodiments, the actuator is permanently anchored to the substrate.
In some embodiments, instead of having the axis of rotation intersecting part of the actuator, where on actuation it could interfere with the substrate, resulting in failure to rotate, the actuator can instead rotate away from the plane of the substrate about some virtual axis of rotation away from the actuator.
Embodiments of the present invention include single and plural-axis rotary motion with anchored and releasable geometries. Potential applications include rotary tweezers; zero insertion force (ZIF) connectors with large contact surface areas; micro-mirror scanning, active optical alignment and beam steering, e.g., for telecom; large angle optical scanners; endoscopy and micro-surgery; MEMS manipulators; and any application in microsystems which requires large angle rotation about an axis.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.